There are a ton of options for that 6-note grouping: just with the limitations of starting with thumb and playing the first two and last two notes in one position, you could do [+ 1 2 3] [+ 1], [+ 1 3] [+ 1 3] (or its cousin [+ 1 2] [+ 1 2], [+ 1] [+ 1 2 3], [+ 1] [+ 1] [+ 1], [+ 1 2 3] [1 3/2], [+ 1 3] [1 2 3], [+ 1] [1 2] [1 2], etc. It's all dependent on musical context and comfort and sometimes a "weird"/unorthodox fingering might be the best solution in one scenario.Thank you to all of you. The most informative part of it is the diversity of approaches.
I also would be interested about your shifting strategy (up and down) in TP. Do you shift "past finger 3" or do you do some kind of "finger replacement"? For example, how would you finger G A B C D E (up) E D C B A G (down) on the G string? Also, where do you place your thumb when playing the other fingers?
For the second question, the thumb is never holding the string down unless it's the playing finger, but I try to always keep a diatonic pitch between + and 1, usually a half step, whole step, or minor third. I like my hand shapes (specifically + 1 2 or + 1 3 since I prefer to usually think of 3 note groupings and I usually stay away from 2-3 fingerings) to reflect the key/chord of the moment. If I'm playing A-B on the G string with 1 and 3 in TP and the key of the moment is A major, I'm not going to leave my thumb parked on G, it's probably going to be on G# (or F# with a minor third between + and 1, but that's entirely contextual) because that's more reflective of the tonality. The exception to this is that in the F–Ab range on the G string (where I'd be using a 1 2 3 fingering on either F-F#-G or F#-G-G#) I'll use a transitional position where my thumb is no longer behind the neck but I don't want to employ thumb position, usually because I'm either doing it for vibrato purposes, I'm transitioning from TP to neck position or vice versa, or I'm staying in neck position except for those couple notes where going fully into TP isn't worth it.